1. Field of the Invention
The present invention relates in general to a heat treatment furnace, and more particularly to a heat treatment furnace which can form uniform diffusion layers and oxide layers on semiconductor wafers throughout the entire heat treatment furnace when performing a heat treatment on the semiconductor wafers and also can suppress the deterioration of the quartz reaction tube used for diffusion of antimony.
2. The Prior Art
A heat treatment such as impurity diffusion or thermal oxidation is an essential means for formation of semiconductor elements, and impurity diffusion and thermal oxidation for various purposes using a heat treatment furnace are conducted in many processes even when manufacturing just one semiconductor device.
FIG. 6 is a schematic cross section of an open-tube type heat treatment furnace. In this figure, a cylindrical reaction tube 1 has a gas supply port 6 on one end connected to an impurity source, not shown here, and a detachable front cap 10 on the other end, i.e. the end with an opening 5. A heater 4 is placed on the circumference of the reaction tube 1 such that it surrounds said reaction tube 1.
When conducting, for example, a diffusion treatment using the heat treatment furnace of the aforementioned configuration, a plurality of semiconductor wafers 2 (hereafter referred to as "wafers") are placed in line on a wafer boat 3 and then put into the reaction tube 1 through the opening 5. The opening 5 of said reaction tube 1 is then closed using the front cap 10. The inside of said reaction tube 1 is then heated to a prescribed temperature of approximately 900.degree.-1300.degree. C., and the carrier gas containing the impurity is supplied through the gas supply port 6. Said carrier gas feeds the impurity onto the wafers 2 as it flows inside the reaction tube 1 before it is discharged outside through the exhaust port 14 provided on said front cap 10. The impurity delivered on the wafers 2 is heated and diffused into the wafers 2.
The temperature distribution in said reaction tube 1 is not uniform. Because of this, although the central portion of the reaction tube 1 where wafers 2 are placed is maintained at the prescribed temperature, the temperature becomes lower towards the ends of the reaction tube 1, i.e. the gas supply port 6 and the opening 5. The fact that the temperature becomes lower towards the ends of the reaction tube 1 causes many problems particularly near the opening 5 end.
For example, in a conventional heat treatment furnace, such as described above, the impurity concentration in the diffusion layer(s) and the thickness of the oxide layer(s) formed on the wafers 2 near the opening 5 are uneven. This is because a conventional current.(A), as shown in FIG. 6, is formed near the opening 5 by the high temperature gas heated in the central portion of the reaction tube 1, which tends to rise, and by the cool gas flowing back into the vicinity of the opening 5 through the exhaust port 14 of the front cap 10.
When diffusing antimony, as an impurity, into the wafers 2, antimony oxides 11 (Sb.sub.2 O.sub.3, Sb.sub.2 O.sub.4, Sb.sub.2 O.sub.5, etc. or composites of these, (hereafter generically referred to as SbO.sub.x) deposit on and adhere to the inner surface of the reaction tube 1 near both ends of the reaction tube 1 where the temperature is relatively low. Near the gas supply port 6, the antimony oxides 11 (SbO.sub.x) simply accumulate on the reaction tube 1. However, near the opening 5 where the temperature can reach about 1200.degree. C., when the reaction tube 1 is made of quartz, the antimony oxides 11 (SbO.sub.x) adhered to the inner surface of said reaction tube 1 accelerate crystallization of the quartz and devitrify the inner surface of said reaction tube 1.
A partially devitrified quartz reaction tube 1 develops cracks due to the difference in the thermal expansion coefficients of crystallized portions and non-crystallized portions. As a result, breakage tends to occur near the opening 5 of said reaction tube 1. This deteriorates not only the reaction tube 1 but also the quality of the wafers 2 which are given the diffusion treatment if the broken pieces of the reaction tube adhere to the wafers 2.
The deterioration of the reaction tube as described above is a problem which mainly occurs when using a reaction tube made of quartz to diffuse antimony. The deterioration of quartz reaction tubes is not a serious problem when diffusing, for example, phosphorus or boron into the wafers.